The invention relates to Coriolis-type mass flowmeters.
In response to the need to measure the quantity of material being delivered through pipelines, numerous types of flowmeters have evolved from a variety of design principles. One of the more widely used types of flowmeters is based on volumetric flow. Volumetric flowmeters are at best inaccurate in determining the quantity of material delivered, where the density of the material varies with temperature of feedstock, where the fluid being pumped through the pipeline is polyphase such as a slurry or where the fluid is non-Newtonian such as mayonnaise or other food products. In addition, the metered delivery of liquid components for chemical reactions, which are in effect mass reactions where proportions are critical, may be poorly served by volumetric flowmeters.
A mass flowmeter, on the other hand, is an instrument that provides a direct indication of the mass, as opposed to volume, of material being transferred through the pipeline. Mass flowmeters measure mass in a moving stream by applying a force to the stream and detecting and measuring some consequence of an acceleration that results from the applied force.
One class of mass measuring flowmeters, referred to as Coriolis-type mass flowmeters, is based on the well-known Coriolis effect. Examples of Coriolis-type mass flowmeters are described in U.S. Pat. No. 4,891,991 to Mattar et al., entitled "Coriolis-Type Mass Flowmeter," issued on Jan. 9, 1990, U.S. Pat. No. 4,911,020 to Thompson, entitled "Coriolis-Type Mass Flowmeter Circuitry," issued on Mar. 27, 1990, U.S. Pat. No. 5,048,350 to Hussain et al., entitled "Electromagnetic Driver and Sensor," issued on Sep. 17, 1991 and U.S. Pat. No. 5,054,326 to Mattar, entitled "Density Compensator for Coriolis-Type Mass Flowmeters," issued on Oct. 8, 1991, all assigned to the assignee of the present invention and incorporated herein by reference in their entirety.
Such Coriolis-type mass flowmeters induce a Coriolis force by oscillating a conduit sinusoidally about a pivot axis orthogonal to the length of the pipe. In such a mass flowmeter, Coriolis forces result due to the flow of mass through the oscillating conduit. The Coriolis reaction force experienced by the flowing mass is transferred to the conduit and is manifested as a deflection or offset of the conduit in the plane of rotation in the direction of the Coriolis force.
A major difficulty in these oscillatory systems is that the deflection due to the Coriolis force is relatively small compared not only to the drive deflection but even to extraneous vibrations. An advantage is that an oscillatory system can employ the inherent bending resiliency of the conduit as a hinge or pivot point for oscillation to obviate the need for separate rotary or flexible joints, and can thereby improve mechanical reliability and durability. Moreover, an oscillatory system allows use of the resonant frequency of vibration of the conduit as the drive frequency, which reduces the drive energy needed.
Energy is supplied to the conduit (or conduits) by a driving mechanism that oscillates the conduit through application of a periodic force. One type of driving mechanism is exemplified by an electromechanical driver that exhibits motion proportional to an applied voltage. In an oscillating flowmeter, the applied voltage is periodic, and is generally sinusoidal. The periods of the input voltage, the resulting driving force, and the motion of the conduit are chosen to match one of the resonant modes of vibration of the conduit. As mentioned above, this reduces the energy needed to sustain oscillation.
When a flowmeter operated with extremely caustic or abrasive fluids for long periods of time, the conduit (or conduits) of the meter can be damaged by wear, causing, in the short term, inaccurate readings and, ultimately, failure of the conduit or tube. Because these flowmeters operate for long periods of time in normal use, catastrophic tube failure is an ever present danger, and, depending on the nature of the process fluid, can cause substantial damage to the facility housing the flowmeter. Accordingly, prevention of tube failure is advantageous and desirable.
The three most likely failure mechanisms in Coriolis flowmeters are listed below. First, flowmeters may fail from corrosion of the tubes due to the flow of caustic process fluids that results in blowout or cracking. Second, flowmeters may fail from erosion of the tubes due to the flow of an abrasive process fluid through the tube that progressively reduces wall thickness until the tube cannot support the process fluid at pressure. Third, in some flowmeters that use vibrating tubes, vibration induced high-cycle fatigue of the tubes may result in tube failure.
All of the failure mechanisms are interrelated to some degree. Corrosion of the tubes would invite fatigue failure (possibly within the normal lifetime of the meter), whereas even minimal fatigue effects might promote corrosion by allowing the process fluid to attack the tubes in a microcracked or stressed area. Further, corrosion and erosion go hand in hand. Tubes that are being eroded by the process fluid would be much more susceptible to corrosion. Such tubes would likely fail in a section that has eroded and subsequently corroded. For the corrosion and erosion mechanisms, the pressure rating of the tube drops until there is a blowout. Pure fatigue failure results in the tube simply breaking. Finally, any impurities in the metal of the tubes could provide a site for corrosion failure.